Aluminum Rotational Mold Finishing

Molds

• In the rotomolding process, the manufacturer introduces powdered, granular or liquid plastic into a hollow mold -- often made from cast or machined aluminum. Slowly rotating the mold around two axes causes the plastic to adhere to the surface of the mold. The rotation is continued while the plastic cools -- at least until it becomes rigid enough to safely remove it from the mold. The rotomolding process enables manufacturers to make hollow parts in one piece and avoids material waste, but it can also entail longer manufacturing times and a limited choice of materials for the designer.

Aluminum Molds

• Most rotational molds are made of either sheet steel or aluminum although sometimes electroformed nickel is employed instead. The aluminum molds have to be thicker than steel molds since aluminum is a better conductor of heat, however, the increased thickness doesn't entail any major drawbacks. Casting is the most common way to make aluminum molds. Sand casting, pressure casting and plaster casting are all viable approaches. Pressure casting is more costly but ensures fewer defects.

Quality

• The quality of the finish required on an aluminum mold depends on the quality of the finish required for the final product. Sometimes the mold can be used as-is without further finishing; if so, pressure casting or plaster casting is the preferred method. Alternatively, the mold can be ground, polished or treated to lend it a different texture depending on the texture needed in the plastic item because the mold will impart its texture to the plastic that adheres to it.

Plastic Finishing

• In many cases, the aluminum mold may also be chemically treated in a way that makes the cooled plastic part easier to remove. Alternatively, manufacturers may add chemicals that help separate the plastic from the metal after each cycle although in this case the mold may need to be periodically cleaned to remove any built-up release agent and thereby avert damage to the finish on the plastic part. In many cases the plastic part must also be altered or finished after molding. It may need to be trimmed, for example, routed or fitted with inserts, depending on the part and its application.